Ceramic carrier have been conventionally used for catalytic converters used for purifying exhaust gas discharged from automobile engines. However, recently, the use of metallic catalyst carrier is increasing because the resistance to heat of the catalyst carrier is high and, further, the pressure loss caused by the catalyst carrier is low and furthermore catalytic converters into which the catalyst carrier are incorporated can be easily mounted on automobiles.
As shown in FIG. 1, a conventional catalyst carrier 1 is manufactured in such a manner that a metallic honeycomb body 2 composed of heat-resistant stainless steel foil containing aluminum is incorporated into an outer cylinder 3 made of metal. As shown in FIG. 2, the metallic honeycomb body 2 is manufactured in such a manner that a piece of strip-like flat foil 5, the thickness of which is approximately 50 μm, and a piece of strip-like corrugated foil 6, which is made by corrugating a piece of strip-like flat foil 5, are put on each other and spirally wound round a winding shaft 8 in the direction of arrow B. On the piece of strip-like corrugated foil 6, a ridge 7 is formed at each wave in the width direction. In the columnar metallic honeycomb body 2 in which the piece of corrugated foil is spirally wound, a large number of vent holes 4 are formed in the axial direction of the columnar body. Catalyst carried is supported in these vent holes 4. In this way, the catalytic converter is composed.
High durability is required of the catalytic converter because it must stand the severe heat cycle given off by the exhaust gas at a high temperature discharged from an engine and further it must stand the intense vibration from the engine. Therefore, in the conventional catalyst carrier 1, the flat foil 5 and the corrugated foil 6 of the metallic honeycomb body 2 are joined to each other at the contact section, and further an outer circumference of the metallic honeycomb body 2 and an inner circumference of the outer cylinder 3 are joined to each other.
Concerning the joining means, there are provided various means such as soldering, diffusion bonding, resistance welding, laser welding and so forth. Diffusion bonding is advantageous because no solder is required and further no specific welding jig is required and, furthermore, diffusion bonding can be appropriately used for mass production.
In general, when diffusion bonding is conducted, members to be joined to each other are made to come close to each other and are heated to high temperature in a high vacuum. In diffusion bonding of the metallic honeycomb body 2 in which pieces of foil are spirally wound, it is impossible to apply surface pressure to the metallic honeycomb body 2 from the outside. Therefore, when back tension is given to the flat foil 5 in the direction of arrow A in the process of winding as shown in FIG. 2, surface pressure can be given, so that the flat foil 5 and the corrugated foil 6 can be made to come close to each other. When the thus formed metallic honeycomb body 2 is heated to high temperature in a high vacuum, the flat foil 5 and the corrugated foil 6 are subjected to diffusion bonding, so that the catalyst carrier 1 can be obtained.
FIG. 3 is a diagram showing temperature curves of the pressures of saturated vapors of iron, chromium and aluminum composing this stainless steel. The pressure of saturated vapor is defined as a value at which the pressure of vapor of a liquid (solid) becomes constant in a vapor phase space when the vapor phase and the liquid phase (solid phase) of the same substance coexist in an equilibrium state. The value of the pressure of a saturated vapor is different for each substance. This value is increased when the temperature is raised. Therefore, when a temperature curve of pressure of saturated vapor is located in an upper portion on the diagram, vaporization tends to occur. Accordingly, aluminum is most vaporized in the materials composing this stainless steel. Due to the foregoing, when the metallic honeycomb body 2 is heated to high temperature in a high vacuum, aluminum is mainly vaporized from stainless steel foil containing aluminum. For the above reasons, when the metallic honeycomb body 2 is heated to high temperature in a high vacuum so as to conduct diffusion bonding on the metallic honeycomb body 2, aluminum is vaporized from the metallic honeycomb body 2, and resistance to oxidation is deteriorated.
However, as shown in FIG. 3, a section of diffusion bonding is not formed in a temperature range of not higher than 1400 K, and a section of diffusion bonding is formed in a temperature range of not lower than 1400 K. This shows that it is necessary for aluminum, the quantity of which is not lower than a predetermined value, to be vaporized in order to form a section of diffusion bonding.
Due to the foregoing, the following are required in order to obtain a metallic honeycomb body of high quality.
(1) Aluminum, the quantity of which is not less than a predetermined value for obtaining a section of diffusion bonding, is vaporized.
(2) Vaporization of aluminum is suppressed to a value not more than a predetermined value so that resistance to oxidation is impaired, that is, it is necessary to control vaporization of aluminum in the process of diffusion bonding.
As a method of controlling vaporization of aluminum in the process of diffusion bonding of the metallic honeycomb body 2, Japanese Unexamined Patent Publication No. 9-215930 discloses the following means. According to the above method, the metallic honeycomb body 2 is covered with a cover having holes and subjected to diffusion bonding. Due to the foregoing, residual oxygen is discharged outside from the holes, and generated aluminum vapor is made to stay close to the metallic honeycomb body 2. According to the above proposal, vaporization of aluminum can be surely suppressed, however, it is necessary to attach the covers to all honeycomb bodies 2, which remarkably deteriorates the productivity. According to this proposal, heat treatment of diffusion bonding is conducted at 1300° C. in a vacuum. When heat treatment is conducted at such a high temperature in a vacuum, aluminum is intensely vaporized, and diffusion bonding cannot be sufficiently done. Therefore, in order to ensure resistance to heat, it is necessary to provide another means instead of the cover described above.
In the same manner, Japanese Unexamined Patent Publication No. 5-168945 discloses another means for controlling vaporization of aluminum in the process of diffusion bonding of the metallic honeycomb body 2. According to this method, a cover is set on the metallic honeycomb body 2, and a clearance is provided between the cover and the outer cylinder. Under the above condition, diffusion bonding is conducted. Due to the foregoing, residual oxygen is discharged outside from the clearance, so that generated aluminum vapor is made to stay close to the metallic honeycomb body 2. According to the above proposal, vaporization of aluminum can be surely suppressed, however, it is necessary to attach the covers to all honeycomb bodies 2, which remarkably deteriorates the productivity.
The applicant filed a patent, by Japanese Unexamined Patent Publication No. 8-38912, in which the following method is proposed. The average roughness (Ra) of a surface of the flat foil and that of a surface of the corrugated foil before diffusion bonding are kept to be not less than 0.001 μm and not more than 0.2 μm, and further the contact width of the flat foil with the corrugated foil is kept to be not less than 30 μm. Due to the foregoing, diffusion bonding can be excellently conducted in all regions of the honeycomb body from the center to the outer circumferential section by giving a back tension, the intensity of which is not higher than a value at which buckling is caused in the vent hole of the honeycomb body, and also by reducing the diameter of the outer cylinder. Also, the applicant filed Japanese Unexamined Patent Publication No. 10-309472 in which the following method is proposed. The contact width of the flat foil with the corrugated foil is kept at a value not less than 5 times of the thickness t of the flat foil and the corrugated foil. The flat foil and the corrugated foil are wound while a back tension, the intensity of which is 0.2 to 1.5 kgf/cm, is being given to the honeycomb body, and diffusion bonding is conducted in a temperature range from 1100 to 1250° C., and it is preferable that diffusion bonding is conducted in a temperature range of vacuum heat treatment corresponding to the average roughness Rac (μm) in the width direction of the plane foil.